OCTOBER 5, 1922
corresponding to the various gliding angles and gliding speeds,
for the assumed wing loading of 2 lbs./sq. ft.
Owing to its popularity for power-driven aircraft and its
good efficiency the section known as RAF 15 might be
expected to be suitable for a glider. It is observed, however,
that the lift coefficients corresponding to the high values of
L/D are small, and possibly the rate of descent will therefore
be fairly high. The first section whose data are tabulated
is the RAF 15, and it will be seen that the highest value of
(L/D)2 X AL IS 75-7 and that this value occurs at an angle
and lift coefficient which correspond to a speed of 36-4 m.p.h.
when the wing loading is 2 lbs. /sq. ft. The rate of descent
is 3-32 ft./sec. For this wing loading the RAF 15 section
gives a landing speed of 27-6 m.p.h.
It was mentioned at the beginning of these notes that the
lowest rate of descent would not necessarily be obtained at
the angle giving the flattest glide, and it will be seen that for
RAF 15 this statement is borne out. The maximum L/D
of this section occurs at 40, but the corresponding gliding
speed is 41-6 m.p.h., which gives a rate of descent of
3-58 ft./sec.
For the section Gottingen No. 441 the lowest rate of descent
is 3-38 ft./sec, but this corresponds to a speed of 30-6 m.p.h.,
and the landing speed is lower, being 22 • I m.p.h. Particulars
of Gottingen No. 441 section were published in our issue of
September 21. It has, we believe, been extensively used on
German gliders, and it will therefore be of interest to examine
several British sections so as to discover whether or not
an even better section can be found.
The section known as airscrew No. 3, the data for which
were published in Reports and Memoranda No. 322, shows
extraordinarily high values of (L/D)2 x AL, the best being
118 as compared with the 75-7 of RAF 15 and 74-4 of the
Gottingen 441. The lowest rate of descent is 2-68 ft./sec,
and the rate does not change greatly over a range of from
20 to 8° incidence. This section is somewhat thin, and would
require a braced wing structure. As a braced monoplane,
however, airscrew No. 3 should be very suitable.
Airscrew No. 4 gives a very slightly higher rate of descent,
and its L/D (or gliding angle) is not quite so good. It is,
however, a much thicker section (maximum thickness 0-127
of the chord), and could be used on a cantilever monoplane.
It gives a landing speed of but 21 -5 m.p.h.
Airscrew No. 5 gives a slightly higher rate of descent, and
its best gliding angle is 1 in 14 • 7. It is, however, a very
thick section, and might be used in the centre of a large-
span wing, end sections of a thinner section being attached
to it.
The foregoing remarks should help to give those who have
not previously given the subject much thought a general idea
of the things to be aimed at in choosing a section for a glider.
It will readily be understood that other considerations than
aerodynamic ones have to be considered. Thus a thin wing
will be heavier per unit of area than a thick wing for the
same strength, and this has to be taken into account in
deciding the respective merits.
As to the actual piloting of a glider, a few general remarks
may not be without interest. If our glider is fitted with
airscrew No. 4 section, and we wish to remain in the air as
long as possible, we would obviously fly the machine at the
speed corresponding to the lowest rate of descent, i.e., at
32 m.p.h. Even in a gusty wind the pilot would try to keep
the machine at this speed ; that is to say, in a gust he would
elevate and in a lull he would push the stick forward. Gliding
is largely a matter of flying at a constant speed, and in this
connection it is of interest to note that Smith and Sons have
already introduced an air-speed indicator which reads from
10 m.p.h. upwards, and is therefore specially suitable for
gliders.
If a pilot wished to cover the longest possible distance, he
would fly at the angle giving the flattest glide, i.e., at maximum
L/D. This applies to flying in still air. In a wind the pilot
would attempt to gain height while gliding into the wind,
and when he felt himself getting outside the region of ascending
air currents he would turn down wind and glide for as long
a distance as possible.
Angle
of
Incidence
(Legs.)
4
5
6
8
—2
0
2
4
2
3
4
6
8
2
3
4
6
2
4
6
8
L/D. &..
R.A.F. 15
(L/D)* (L/D)*.
X ftL.
(Absolute.)
17-1
16-7
16-1
i4'3
15-2
14-6
13-3
12-2
19-7
19-4
18-3
16-5
14-8
17-5
17-5
17-1
15-5
14-7
140
13-0
12-0
•226
•260
•296
• 369
292 66-o
279 72-5
259 75-7
202 74•5
Landing speed .
Gottingen No. 441
•275
•34
•42
•48
231 63-5
213 72-5
177 74-4
149 71-6
Landing speed .
Airscrew No. 3
•274
•313
•35i
•425
•490
388 106-4
376 118-0
335 "8-o
272 116-0
219 107-4
Landing speed
Airscrew No. 4
•308
•345
•384
•453
306 94-5
306 106-0
292 1120
240 109-0
Landing speed
Airscrew No. 5
•39i
•462
•536
•599
216 84-5
196 90-5
169 90-6
144 863
Landing speed
V.
(m.p.h.)
41 -6
38-9
36-4
32-6
. 27-6
37-7
34-o
30-6
28-6
.22-1
37-8
35-4
33-4
30-3
28-3
•• 25-3
35-7
33-5
32-0
29-4
• • 21-5
3i-7
29-1
27-1
25-6
. . 22-2
Vv.
ft./s.
3-58
3-42
3-32
3-36
3-65
3-42
3-38
3'44
2-82
2-68
2-68
2 "JO
2-8l
3-o
2-83
2-75
2-79
3-17
3-06
3-°5
3-14
THE NEW DE HAVILLAND GLIDER
Parasol Monoplane with Wire Bracing
THE first of the two monoplane gliders which are being
constructed at the Stag Lane works of the de Havilland
Aircraft Co. will be finished this week, and it is hoped that
some preliminary test flights may be made during the next
week or so. On Monday we paid a visit to the D.H. works,
and the glider was then erected and the bracing wires of
the wings were being rigged and finishing touches given to
various minor parts. At present the machine is fitted with
a vee under-carriage, but probably later on, when the pilot
has got used to the machine, a modified form, fitted inside
the fuselage, will replace the present tubular vees so -as to
save resistance.
From the accompanying scale drawings it will be seen that
the de Havilland glider is characterised by a parasol mono
plane wing of very high aspect ratio (n to 1), and that in
spite of the fact that the wing has external wire bracing
the appearance is very clean. The wing section used is
R.A.F. 15, but a slight departure from the standard section
has been made by stepping down slightly the ordinates of
the top surface. This has been done in order to reduce spar
weight, as it was found that with the spar section employed
(I-section) the standard section would give rather too heavy
spars. Both front and rear spars are of spruce, and the ribs
are built up of spruce flanges, approximately £ in. square,
the form of which is preserved by webs or distance pieces
tacked to one side of the spruce flanges, as shown in one of
our sketches. ff
The internal bracing is in the form of small-gauge piano
wire, and the use of wire strainers has been avoided by using
U-bolts passing horizontally through the spars. The system
of bracing is, however, different from that employed in the
old Bleriots, inasmuch as alternate bays have the drag wires
anchored to ordinary wiring plates. Thus the trueing-up
is not quite the work of art it used to be in the Bleriotsjof
old.
The monoplane wing is very simply mounted on the fuselage
by two eye bolts engaging with corresponding forked end
bolts at the apices of two formers or bulkheads rising up
from the top longerons of the fuselage proper. The wing
thus rests on the fuselage at two points only, and these on
the centre line. The bracing is therefore relied upon to
maintain the wing in its proper transverse position relatively
to the fuselage.
As far as the stresses in the wing are concerned, the spars
may be regarded as continuous beams, as the two end
sections are not pin-jointed to the large-span centre section.